The present invention relates generally to tracking systems that use magnetic fields such as for surgical interventions, and other medical procedures. More particularly, the present invention relates to apparatus and techniques for determining magnetic field distortion in such systems.
Tracking systems have been used to provide an operator (e.g., a physician) with information to assist in the precise and rapid positioning of a medical (e.g., surgical) device in a patient's body. In general, an image is displayed for the operator that includes a visualization of the patient's anatomy with an icon or image representing the device superimposed thereon. As the device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device coordinates. The image of the patient's anatomy may be generated either prior to or during the medical or surgical procedure. Moreover, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound, may be utilized to provide the basic image in which the device tracking is displayed.
To determine device location, tracking systems have utilized electromagnetic (EM) fields. During these procedures, signals are transmitted from one or more EM transmitters to one or more EM receivers. In one example, an EM receiver is mounted in an operative end of the device. In general, the EM transmitters generate an electromagnetic field that is detected by the EM receivers and then processed to determine the device location, for example, the position and orientation, including the X, Y and Z coordinates and the roll, pitch and yaw angles.
However, as those of ordinary skill in the art appreciate, the presence of field distorting objects may result in distortions in the magnetic field emitted from the EM transmitters and thereby change the magnitude and direction of this field. For example, the presence of a signal from another source, the magnetic field of the eddy current in a conductive object, or the field distorting effect of a ferro-magnetic object can result in these distortions. Unless compensated for, these distortions will result in error in the determined location of the device. One source of magnetic field distortions may be the equipment utilized in the tracking system itself. For example, certain tracking systems include a fixture containing one or more EM sensors that are attached to an imaging system, such to the C-arm of an X-ray fluoroscopy system. As those of ordinary skill in the art will appreciate, these imaging systems typically include conducting objects (e.g., the C-arm) that result in the above-described field distortions. To compensate for this known distortion, a distortion map is generally created for each tracking system during the factory calibration process. This distortion map is used by the tracking system to compensate for this known distorting effect during the medical procedure.
An exemplary technique for creating the distortion map for a tracking system that includes an X-ray fluoroscopy system containing a C-arm, involves use of a precision robot. An EM transmitter is attached to an arm of the robot and moved to numerous points in space within the navigated volume. At each point, signals from the EM transmitter are detected by one or more EM receivers and then processed to determine a measured location of the transmitter with respect to the receiver, which is rigidly fixed to the C-arm of the X-ray fluoroscopy system. Because a precision robot is used, the real world location of the transmitter at each sampled point in the navigated volume is known. Accordingly, the measured location of the device detected by the receivers is compared to the transmitter's real world location to generate the distortion map that is used by the tracking system. By way of example, the distortion map may cross-reference the measured transmitter location with the real world transmitter location. However, to generate a complete distortion map, the transmitter must be positioned at thousands of points within the navigated volume. This process of collecting thousands of data points is time consuming and resource intensive. Moreover, extra time may be required to allow for the robot arm to stabilize at each point, and extreme care must be used to ensure that the system is not disturbed during data acquisition.
In addition to the tracking system itself, field distorting objects also may be present in the clinical environment where the tracking system is used. However, the impact of these field distorting objects on the magnetic field in the clinical environment is generally not known, and the field distorting objects are frequently transient. Techniques for detecting distorting objects during medical procedures have been developed. One such technique utilizes two sensors rigidly mounted at a known fixed distance on the medical device to detect uniform distortions in the area of the medical device. However, these techniques only detect field distortions in the immediate vicinity of the two sensors and do not convey the extent of field distortions in the larger navigated volume.
Accordingly, there is a need for an improved technique for detecting and correcting for magnetic field distortions. Particularly, there is a need for a technique that detects magnetic field distortions in and around a tracking system so that these distortions can be accounted for in the clinical environment.